Answers:The definition of a strong acid is one that dissociates completely in water. That is 100% of the molecules separate to form ions, and none of the acid remains a dissolved molecule.
HCl, HBr, HI, HNO3, HClO4 and H2SO4 (1st H only) are the common strong acids.
When 100 molecules of HCl(g) are dissolved in water, then there will be 100 H+ ions and 100 Cl- ions and no molecules of HCl
When 100 molecules of HF, a weak acid, are dissolved in water, then there will be 1 H+ and 1 F- and 99 HF molecules that don't separate, assuming 1% dissociation. Actually, for HF there will be even fewer H+ and F- ions since it is much less than 1% dissociated.
H2SO4 is more complicated. Sulfuric acid is a strong acid, but only for the first hydrogen that comes off. What is left, HSO4-, is a weak acid, and does not dissociate completely.